The present invention relates to electric motors and, more particularly, to controlling the speed of an electric motor.
Currently, electric motor speed controls employ a variety of electro-mechanical or expensive electronic techniques in an attempt to maintain a desired motor speed. Existing electro-mechanical motor speed control techniques do not provide the capability of detecting a stalled motor condition to remove the power to the motor to prevent personal injury or equipment damage. Also, existing electro-mechanical techniques for controlling motor speed are unable to precisely detect motor speed changes because response times of the mechanisms are inherently long.
Known electronic techniques for controlling motor speed either approximate the speed by use of a charged capacitor or by expensive pickup devices. While some of the present electronic motor speed controls monitor the current through the motor and remove the power at some predetermined current level, this does not allow a programmed time period for the motor to attempt to overcome the obstacle by increasing the power to the motor. In addition, low level current detection results in unwanted power interruptions.
The ability to control the increasing and decreasing rate of speed is also not available with the present techniques. This is a disadvantage in that it prevents the motor and the control from being integrated for maximum power efficiency and does not allow the motor's speed changes to be mated to the application. Some alternating current electric motor speed controls also have greatly unbalanced phase angles through positive and negative cycles. This creates a DC power component in the motor that is both inefficient and could cause motor damage. Finally, present alternating current electric motor speed controls lack a provision for eliminating brush bounce power loss when the thyristor is turned off too soon. Premature firing of the thyristor can also result in power loss if the inductive current, related to the voltage cycle, has not ceased when thyristor firing is attempted.
It is seen then that there exists a need for a technique for controlling an electric motor that overcomes the disadvantages encountered in current electric motor speed control systems.